Question

A sample of 12 M Concentrated hydrochloric acid has a density 1.2 M gL^-1 calculate the molality.

Answer 1

Given:

Molarity = 12 M HCl

density of solution = 1.2 g L^-1

In 12 M HCl solution, there are 12 moles of HCl in 1 litre of the solution.

Molality = `"no of moles of solute"/"mass of solvent (in kg )"`

Calculate mass of water(solvent)

mass of 1 litre HCl solution = density × volume

= 1.2 × gmL^-1 × 1000 mL = 1200 g

mass of HCl = no. of moles of HCl × molar mass of HCl

= 12 mol × 36.5 g mol^-1

= 438 g.

mass of water = mass of HCl solution – mass of HCl

mass of water = 1200 – 438 = 762 g

molality(m) = `12/0.762` = 15.75 m